1. Field of the Invention
The present invention relates to an image data processing apparatus, and, more particularly, to an image data processing apparatus that performs pixel quantity conversion and error diffusion processings on image data.
2. Description of the Related Art
Information machines, such as facsimile machines, copying machines and optical code readers (OCR), each comprise a line sensor like a charge coupled device (CCD) or a contact type sensor and an image data processing apparatus connected to the line sensor. The processing apparatus receives an analog image signal supplied from the line sensor and converts that signal to binary image data. A modem installed in a facsimile machine sends such binary image data to a destination facsimnile machine. A copying machine prints binary imaged data on a printer. An OCR sends out binary image data to a computer.
To increase the data transfer speed in facsimile machines, image data having a relatively low pixel quantity (e.g., 200 dpi) is used. To improve the image quality in copying machines, image data having a relatively high pixel quantity (e.g., 400 dpi) is used.
There are information machines, which have a communication function and a printer function, are connected to a computer. An image data processing apparatus incorporated in such an information machine is configured to convert the pixel quantity of data in accordance with a communications device, a printer or a computer, when the pixel quantity of the line sensor (e.g., 300 dpi) is lower than the pixel quantity of a printer (e.g., 400 dpi), the image data processing apparatus converts the pixel quantity of data to match the pixel quantity of the printer to prevent the printed image from being reduced. In the case of a copying machine, for example, the processing apparatus outputs binary data of the same pixel two by two so that the number of pixels is doubled to provide a high pixel quantity, Successively outputting two same pixel data however makes printed characters thicker. Thicker characters are harder to see.
The computer supplies image data having a relatively high pixel quantity (e.g., 800 dpi) to the processing apparatus, In the case of outputting such image data onto a printer, binary data is thinned every other pixel in order to halve the number of pixels, thus ensuring a low pixel quantity. This technique however may change the width of a printed line and may make a thin line disappear. As one example, when the binary data of the single center pixel of a line consisting of three pixels is thinned, that line is printed as a line consisting of two pixels. When the binary data of both side pixels of a line consisting of three pixels is thinned, that line is printed as a line consisting of one pixel. As another example, a line consisting of one pixel may disappear when thinned.
An error diffusion method is employed to naturally display a multi gradation image using a binary display system or binary (white and black) printing system with high quality. The error diffusion method is known as a high-performance pseudo gradation processing technique. The error diffusion method miacroscopically expresses the gradation by computing a difference or error value between pixel data and a predetermined threshold value and adding or subtracting the error value to or from pixel data of those pixels around the pixel to be displayed. The threshold value is predetermined to determine the on/off (white/black) of the display and printing, The error value is computed for each pixel data in one line and is temporarily stored. The error value is added to or-subtracted from pixel data of the corresponding pixel in the next line and pixel data of those pixels around that pixel. The image data resulting from the addition or subtraction is compared with a threshold value to determine if it is white or black.
Pixel quantity conversion is carried out prior to the error diffusion process. Therefore, the error diffusion process uses individual pixel data that have been generated by the pixel quantity conversion. when image data having a relatively high pixel quantity is generated by the pixel quantity conversion, it is necessary to store a greater amount of error values data than image data that has not undergone pixel quantity conversion. This requires a large-capacity memory and inevitably enlarges the apparatus.